Various apparatus and methods for continuously casting two sided metal strip utilizing opposed moving chilled surfaces, are known or have been suggested in the past. In such instances, two confronting, moving surfaces are employed. In a "Bessemer" machine (U.S. Pat. No. 49053), a pair of confronting but spaced apart rolls define the moving surfaces. The axes of the rolls are typically parallel and horizontally positioned and the roll surfaces form a nip where the rolls are closest to each other which defines the casting thickness. Molten metal contained between upper portions of the roll and side dams above the nip, freezes on the chilled periphery of the rolls.
In theory, a continuous solidified strip of metal is discharged vertically downward from the nip of the rolls. It has been found, however, that the process of using rolls and side dams for containing the pool can be very difficult to control. In particular, in the fixed gap mode of operation, (i.e., where the roll center to center distance is fixed) if sufficient contact time is not provided between the metal being solidified and the chilled surfaces, "break out" can occur in which molten metal in the center of the strip ruptures through the hardened outer layer. If the contact time is excessive, the rolls may jam because the total thickness of material solidified on each of the rolls is greater than the nip dimension. In addition, freezing of material on the side dams causes jams and/or other process problems.
The Bessemer-type machine is considered a "converging gap" type machine (as opposed to a "constant gap" machine) since the pool of molten metal carried above the confronting rolls has a transverse dimension that decreases as the nip is approached. In a Bessemer machine, the contact time is determined by the dimension of the rolls and their speed and the pool depth.
In an effort to overcome some of the difficulties of the Bessemer machine, an "inside-the-ring" (ITR) type machine has been suggested in the past in which a large rotating vertical ring contains a pool of molten metal at the bottom. As the ring rotates, molten metal freezes on an inside surface to form a strip of material that is discharged spirally from the ring. The ring normally has cooled metal side dams which contain the pool of molten metal. To make the process two-sided, a roll or drum is rotatable with the ring and defines a gap or nip between itself and the inside of the ring. An example of such an apparatus is shown and described in U.S. Pat. No. 3,773,102. In this type of machine, "ear loss" that is, the material which is cast against the side dams and must later be trimmed becomes a concern. It should be appreciated that molten metal freezes on any chilled surface and in the case of an "ITR" machine, material can be expected to solidify on the side dams and the sides of the drum. Generally the "ears" can be cut from the strip and re-used as scrap metal.
Constant gap strip casting machines (such as the Hazelett twin belt machine which is well known in the industry) do not in general have an ear problem in that they cast strip of a rectangular rather than a channel-shaped cross section. In the typical constant gap machine liquid metal fills the gap at the input end of the machine and as the metal moves down the machine and freezing progresses from the walls, the central core of liquid metal gradually decreases from the full thickness of the constant gap to zero. Such machines use constant thickness blocking means to keep the liquid from running out at the ends; such blocks may run along with the moving casting surfaces.
Generally the most serious problem encountered with constant gap machines is providing a means for introducing the molten metal into the casting gap. Since in all constant gap machines, a gap corresponding to the final casting thickness is defined between the two casting surfaces, for small thickness strip material, very little access is provided for introducing molten metal.
Several methods have been used for preventing the metal from running out at the ends in converging gap machines. Although attempts have been made to contain the metal with insulators which are supposed to operate at a high temperature so that no metal is cast against them, the general method of interest here involves containing the converging gap with casting surfaces. Two such methods are exemplified by the inside-the-ring machine (U.S. Pat. No. 3,773,102) and the Schloemann drum-belt machine (U.S. Pat. No. 3,627,025). In both of these a pool of metal is restrained at the sides by metal (or coated metal) side dams which preferably move contiguously with the ring or belt of the machine. In the Schloemann machines, the ears are straight up (i.e., at right angles to the strip). In the ITR machine the ears project upward at some greater angle than 90.degree. to the strip. However, in both of these machines, a gap is provided between the end of the drum and the ring side dam, this gap must be at least wide enough to accommodate the thickness of metal that is frozen on the drum ends and on the side dams. This gap is typically open ended at the top.
Another machine utilizing cast in ears as an edge restraint is seen in U.S. Pat. No. 2,450,428 (Hazelett) and features a drum with rounded ends proximate to and forming a nip with either the outside or the inside of a large ring that is fitted with side dams which cast ears of arcuate shape. Here the ears are arcuate and taper to zero thickness at the top. It is noted that the side dams of the ring each touch the adjacent rounded end of the drum at essentially only one point. This point is a point of tangency of a circle on the drum and another circle on the ring.
In all of the converging gap designs cited above, the depth of the pool is at most the height of the side dam of the ring, and in the case of the Hazelett roll outside the ring machine it is less.
Another important consideration is the productivity of a given machine. As indicated above, the speed at which a machine can produce a solidified strip is a function of "contact length" of the molten metal with the chilled surface or surfaces. It is well known from actual experiments that the thickness of casting that builds up against a chill surface varies at least approximately according to the relation
x=K.sqroot.t-B where PA0 x=inches of casting thickness PA0 t=seconds of immersion time PA0 K and B are constants depending on the parameters of the system (materials, temperatures, etc.)
It follows that if a given thickness is to be cast, a certain immersion time is required to cast it. This time may be realized for example by either a short immersion length in a slowly moving machine or a long immersion length in a fast moving machine. Obviously machines with long immersion lengths are faster and more productive, and an otherwise small machine with a long immersion length is to be preferred from a first cost and a productivity standpoint.
For converging gap machines with appreciable contact time, the formation of ears becomes a serious problem. It should be appreciated that the increased contact time which allows a greater productivity of strip also increases the time during which the ears can be formed and hence greater ear thickness may result. In all inside the ring or belt machines of converging gap design where an open pool is employed (e.g. the ITR or the Schloemann machines), the ear height is at least as great as the depth of the open pool.